Prior to the present invention, dihydric phenol, such as bisphenol-A (BPA), was phosgenated in a reactor under interfacial reaction conditions resulting in the production of products, such as BPA bischloroformates (BCF), oligomeric carbonate chloroformates, or polycarbonate. There was generally utilized aqueous caustic soda and organic solvent, such as methylene chloride, and the phosgenation reaction was conducted in either a batch or continuous reactor.
Although satisfactory results have been generally achieved during interfacial phosgenation of dihydric phenols, it has been found more difficult to achieve satisfactory results under conditions in which dihydric phenol is fed continuously to the phosgenation reaction as compared to batch-wise conditions. A typical BCF phosgenation reaction mixture can include products having the formulas, ##STR1## where m can have a value of 1-15, m' is a whole number equal to 0 or an integer having a value of at least 1, and R is a divalent organic radical, such as 2,2-diphenylpropane. A polycarbonate phosgenation mixture can include products having the formula, ##STR2## where R is as previously defined and n can have a value of at least 25.
In determining the stoichiometry between dihydric phenol and phosgene gas in the presence of an aqueous phase, it has been found that wasteful hydrolysis of phosgene can occur. Experience has shown that undue amounts of water can result in excessive phosgene hydrolysis effecting a reduction in the yields of BCF or polycarbonate reaction products per unit of phosgene reacted.
The degree of phosgene hydrolysis also influences the required amount of caustic soda. As a result, there are severe constraints on the amount of each component to be added to the mixture. In particular, BPA is a solid at normal conditions and it is difficult to add it both continuously and consistently, particularly if the rates of addition of other components change. Some of the forms which BPA can be continuously dispensed are:
a. as a dry powder from a hopper using an auger feed screw,
b. as an aqueous slurry,
c. as a slurry in organic solvent such methylene chloride, and
d. as an aqueous solution with NaOH (about 1 mol of BPA per liter of aqueous NaOH having pH .about.11).
For batch addition, dry powder can be pre-added to the reactor; however, it is difficult to perform this addition continuously as voids form in the powder which allow phosgene to back-flow into the BPA hopper, thus creating a hazard. Further, the powder flow rate is neither constant, not consistent in this case.
The slurry concept, whether in an aqueous or organic medium, is very difficult to maintain; density differences between BPA (1.18 g/ml), water (1 g/ml), and methylene chloride (1.3 g/ml) allow separation to readily occur so that monitoring of the delivered slurry is a requisite to ensure constant and consistent flow of the BPA phase.
The use of a homogeneous aqueous solution of BPA in NaOH introduces an excessive quantity of water into the reactor so that phosgene hydrolysis is enhanced.
The present invention is based on our discovery that a stable or near-stable suspension, resulting from the agitation of a mixture of BPA and an alkali metal hydroxide in water, as defined hereinafter, can provide a method of continuously adding a dihydric phenol, such as BPA, into a phosgenation process that has the following benefits
(i) ease of metering dihydric phenol and maintaining proper dihydric phenol weight proportions with respect to other reaction components,
(ii) ease of adjusting dihydric phenol flow rate to changes in phosgene flow rate,
(iii) reducing the proportion of water entering the reactor and thereby minimizing the rate of phosgene hydrolysis, and
(iv) reducing the hazardous back-flow of phosgene into phosgene-free areas.
As defined hereinafter, the term "stable suspension" means a substantially opaque plural phase material capable of assuming the shape of its container up to a free interface while substantially maintaining its initial appearance without phase separation for a period of at least 30 minutes, under atmospheric conditions, which results from the agitation of a mixture consisting essentially of water, alkali hydroxide and dihydric phenol having a proportion of from about 0.01 to 4 moles of alkali hydroxide, per mole of dihydric phenol, and sufficient water to provide a substantially homogeneous fluid. Preferably, the stable suspension of the present invention comprises by weight
(A) 47.9-52.0% bisphenol-A,
(B) 52.0-47.9% water, and
(C) 0.01-0.2% of alkali metal hydroxide.
The proportion of alkali metal hydroxide should be sufficient to avoid foaming during agitation, and provide a pH of up to about 11. The mixture is stable after an extended shelf period. However, to insure uniformity during use, it is preferred to mildly agitate the suspension.
In instances where the stable suspension can be reacted in 8 hours or less after formulation, the following composition will provide effective results:
(D) about 40-70% water,
(E) about 8 to 16% alkali metal hydroxide, and
(F) about 20-50% BPA,
in which higher proportions of water lead to longer periods for which the mixture remains fluid.